<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>京东三角</title>
    <style>
        .box1{
            position: absolute;
            height: 0;
            width: 0;
            border: solid transparent;
            border-bottom: 10px solid dodgerblue;
            left: 100px;
            top: -10px;
        }
        .box2{
            position: relative;
            height: 200px;
            width: 200px;
            margin-left: 300px;
            margin-top: 300px;
            background-color: deepskyblue;
        }
    </style>
</head>
<body>

<div class="box2">
    <div class="box1"></div>
</div>

</body>
</html>